Filters may be used on power lines for the mitigation of electromagnetic noise, or earth leakage current, which may be generated by switching frequencies, for example.
For instance, on motor drive output lines, earth leakage current can result in a rapid degradation of motor bearings.
In many applications, a system switching frequency exists in the range of 1 KHz to 150 kHz. In the case of motor drives, switching frequencies are typically in the range of 1 kHz to 16 kHz and in the case of power supplies, UPS and power conversion they are typically in the range of 10 kHz to 200 kHz.
It is desirable to find efficient ways to remove these disturbing frequencies, and associated harmonics, from both input and output lines.
Conventional filter solutions for mitigating electromagnetic noise on power lines, which are generated for example by the switching frequencies, are passive filters constructed with inductive and capacitive elements and resistors.
Power electronic equipment may employ line filtering that is constructed with such inductive and capacitive elements in order to attenuate line disturbances. The impedance of these passive elements is frequency dependent. In order to achieve the desired attenuation, the passive elements are often large and expensive, especially at low frequencies.
More compact filter solutions are desired, in particular in the frequency range of 1 kHz to 150 kHz where, in addition to switching frequencies, further phenomena in the field of electromagnetic compatibility (EMC) exist such as: spurious tripping of residual current devices (RCDs), disturbance to power line communications, and more generally earth leakage current.
Suitable filters for this purpose may either be built to be ever larger and more expensive; or where this becomes too extreme, the problem is ignored altogether.
WO 2014/048471 A1 describes an EMC filtering device comprising an active filtering section and a passive bandwidth filtering section. The passive and active parts of the filter are mutually beneficial such that the resultant filter has a hybrid nature, is small, and has a lower cost than a passive filter solution alone.